1. Field of the Invention
The present invention relates to a manufacturing method of an optical semiconductor integrated circuit device with a photodiode and intends to eliminate the variation of a film thickness of an insulating layer laminated on the photodiode and to improve the sensitivity of the photodiode.
2. Description of the Related Art
An optical semiconductor integrated circuit device that is formed monolithic by integrating a photodetector and a periphery circuit is different from one in which a photodetector and a circuit element are separately prepared and formed into a hybrid IC. In an optical semiconductor device, cost reduction can be expected. Furthermore, the optical semiconductor device is advantageous in that it is resistant against noise due to an external electromagnetic field.
In an existing optical semiconductor device with a built-in photodiode, for instance, an impurity is diffused on a surface of an N-type substrate to form a P-type semiconductor layer. Subsequently, outside of the neighborhood of a portion where a PN junction is exposed on a surface, a SiO2 film and a SiO3N4 film are alternately stacked three layers each to form a highly reflective film. On the other hand, in the surroundings of a light-receiving portion, over an entire surface except for a contact hole that brings a P-type semiconductor layer and a P-type electrode into contact, a SiO3N4 film is formed to form a photodiode (patent literature 1).
Furthermore, in an existing optical semiconductor device with a built-in photodiode, for instance, on a P-type substrate a non-doped first epitaxial layer and an N-type second epitaxial layer are laminated. In island regions partitioned with isolation regions, a photodiode, a capacitor element and an NPN transistor are formed respectively. In a photodiode formation region, on a surface of the second epitaxial layer, a surface protective film is formed. At this time, a silicon oxide film, a polyimide base interlayer insulating film and a jacket coat on a surface protective film are removed (patent literature 2).
[Patent literature 1] JP-A No. 03-206671 (page 2 and FIG. 1)
[Patent literature 2] JP-A No. 2001-320078 (pages 3 to 5 and FIG. 1)
As mentioned above, in the patent literature 1, on an N-type substrate, only a photodiode element is formed. In addition, even on a surface of the substrate, a single layer wiring structure is formed and an anti-reflection film is exposed.
However, for instance, in an optical semiconductor integrated circuit device having a built-in photodiode, on a surface of a semiconductor layer on which a photodiode element and so on are formed, a multi-layered wiring layer is formed. In the optical semiconductor integrated circuit device, owing to the wiring layer, the respective elements are electrically connected. Accordingly, the respective layers are necessary to be insulated from each other, and, as the insulating layer, a silicon oxide film and BPSG (Boron Phospho Silicate Glass) film made of an inorganic material or a polyimide film made of an organic material are used. When a wiring layer is formed in each of the respective layers, a flatness of the insulating layer is maintained by use of an SOG (Spin On Glass) film or the like.
That is, in the existing optical semiconductor integrated circuit device, owing to the formation of a multi-layered wiring layer, the variation is caused in a film thickness of the insulating layer. In particular, on the photodiode formation region, owing to the variation of the insulating layer, the reflectance is different depending on a position where light enters. As a result, there is a problem in that the fluctuation in the sensitivity of a photodiode is caused accordingly.
On the other hand, in the patent literature 2, in the optical semiconductor integrated circuit device with a built-in photodiode, on a photodiode formation region, only a single layer film of a silicon nitride film is coated as a surface protective film.
However, in the invention in patent literature 2, the silicon nitride film is used as an etching stopper film when the insulating film is wet-etched. By use of wet etching, the insulating film is removed. Accordingly, when the insulating film is removed owing to the wet etching, the etching proceeds in a horizontal direction to a surface of the substrate. As a result, in wet etching, it is difficult to etch into a desired structure, resulting in causing a problem in that the processing accuracy is poor.
Furthermore, when wet etching is used to remove, the etching rates in a horizontal direction and in a depth direction are substantially same. Accordingly, in wet etching, a miniaturization process is applied with difficulty; that is, there is a problem in that wet etching cannot cope with recent super high integration.